The Prime Minister has warned that if the United States is defeated in Iraq it will make handling nations such as North Korea more difficult.

John Howard says if the US pulls its combat troops out of Iraq by March next year it would be a defeat for the Americans.

Mr Howard has weathered criticism from the Opposition, from within his own party and from a bemused US Democratic Party, after he remarked at the weekend that Al Qaeda would be praying for US presidential hopeful Barack Obama to win the 2008 election.

The Opposition says the comments have damaged the US-Australian alliance, and Liberal backbencher Mal Washer says Mr Howard may have gone too far.

Senator Obama, meanwhile, said the Prime Minister should send another 20,000 Australians to Iraq before his criticism could be taken seriously.

But Mr Howard has refused to apologise.

He has told the ABC's 7.30 Report the idea of an early US withdrawal worries him because it would have implications for Australia's national interests.

"My deep concern is that if America is defeated in Iraq a humiliated, enfeebled America might withdraw its interests in our part of the world," he said.

"And it would be a catastrophe for the West if America were defeated in Iraq."

Mr Rudd has challenged Mr Howard to a televised debate on Australia's Iraq policy, but Mr Howard has declined.

He told the 7.30 Report the idea of a televised debate was a well-known ploy by opposition leaders who did not want to reveal their own policies.